Reporting from Los Angeles and Washington — In cutting off access to thousands of websites for a day, the tech industry flexed its political muscle with a don't-mess-with-the-Web campaign that highlighted its vast reach and how indispensable the Internet has become.

The sweeping blackout to protest federal anti-piracy bills sparked frustration and confusion Wednesday but had its intended effect — disrupting the usual flow of the Internet while mobilizing opposition among online users and lawmakers.

More than 10,000 websites participated in the strike against the Stop Online Piracy Act and the Protect Intellectual Property Act, bills that opponents say could lead to censorship online and force some websites out of business. Some, including Wikipedia, Reddit and Boing Boing, shut down for the day, while others such as Craigslist and Google protested by blacking out parts of their sites and urging users to sign online petitions and contact members of Congress.

Shortly after midday, Google said 4.5 million people had signed its petition. Meanwhile, Wikipedia said 5.5 million people had clicked through the blackout message on its home page for information on how to contact their local lawmakers.

"We're pretty staggered by that number," said Jay Walsh, a spokesman for the Wikimedia Foundation. He said the site was advising users to call lawmakers instead of emailing them after getting reports that congressional inboxes were flooded and servers were facing capacity issues.

The impact of Wednesday's action raised the possibility of a bigger and broader Internet strike that could lead to a virtual information blackout. The biggest Internet companies such as Google and Facebook did not shut down — they could have lost millions of dollars in advertising revenue — but expressed their support online.

"Technology has grown as a part of our lives and the companies now have something of value that they can withhold in terms of services, which is a shift in the overall political landscape," said Colin Gillis, a technology analyst at BGC Financial. "Is this spawning a new level of activism? I'd say absolutely yes."

Despite unprecedented publicity ahead of the strike, many Internet users were caught off guard.

"I support what they're doing, but to be honest, I would have preferred to see more what Google did: Leave the service available but make a point," said Burbank resident Robert Rose, 45, who was irritated when he couldn't access some of his favorite websites. The marketing consultant vented on Twitter, writing "BlackoutWorking."

With millions flooding Capitol Hill with emails and calls, some supporters of the legislation publicly backed away.

Sens. Orrin G. Hatch (R-Utah), Marco Rubio (R-Fla.) and Roy Blunt (R-Mo.) withdrew as co-sponsors of the Senate bill. Meanwhile, Reps. Lee Terry (R-Neb.) and Ben Quayle (R-Ariz.) said they were pulling their names from the companion House bill. Some other lawmakers also said they would not support the bills, endangering the push to quickly pass the legislation early this year.

Backed by Hollywood and media activists, the two bills aim to crack down on foreign websites that traffic in pirated movies, music and counterfeit goods. But Web companies argue that the proposed legislation is about more than piracy and digital copyright protection. They say the broad language of the bills would thwart free speech online and could stifle the Internet economy, hurt the creative process and drive up legal costs.

"Given the legitimate vocal concerns, it is imperative that we take a step back to allow everyone to come together and find a reasonable solution," said Hatch, who had been a strong early supporter before the backlash. He called for Senate Majority Leader Harry Reid (D-Nev.) to back off plans to hold a key procedural vote on the bill Tuesday.

Some senators' websites were inaccessible at times Wednesday. And the House saw double its normal Web traffic, said Dan Weiser, a spokesman for the Office of the Chief Administrative Officer of the House.

In the offices of Rep. Dan Lungren (R-Gold River), the phones were chirping every couple of minutes Wednesday afternoon.

"Every time the phone rings, it's SOPA," Lungren spokesman Brian Kaveney said. One staffer answered about 50 calls alone on the issue Wednesday, dutifully writing down each person's name, ZIP Code and viewpoint, with a promise to forward the information to Lungren.

The office received about 100 calls by midafternoon on SOPA, five times the number of the previous day. Almost all callers opposed the bill, but a few simply sought Lungren's position. He supports the goals of cracking down on foreign piracy websites but has problems with the legislation and wants to slow down the process.